In an electronic device in which a plurality of conductor planes are present, an electromagnetic wave is generated as a result of, for example, induction of a magnetic field by electric current flowing into a circuit at a time of switching of a digital circuit, or induction of an electric field by voltage fluctuation occurring at a time of the switching. Such an electromagnetic wave becomes into electromagnetic noise propagating through a parallel plate line constituted by the conductor planes. Such electromagnetic noise causes problems of destabilizing the circuit operation, deteriorating the wireless performance of the device, or the like. It means that the circuit stability and the device's wireless performance can be improved by establishing technology for suppressing the electromagnetic noise.
Conventional methods having been used for solving the above-described problems include a method of inserting a decoupling capacitor between the conductor planes, a method of avoiding formation of a large island-shaped conductor plane, and the like. However, these methods have the following problems, respectively.
In the method using a decoupling capacitor, it is difficult to achieve a self-resonant frequency as high as hundreds of MHz owing to an unavoidable parasitic inductance of the capacitor. Therefore, in general, the method using a decoupling capacitor can be applied for only frequencies up to about hundreds of MHz, and cannot cope with a higher frequency range such as that used in recent wireless communication (for example the 2.4 GHz band or the 5.2 GHz band).
The method of avoiding formation of a large island-shaped conductor plane is based on shifting the resonance of a conductor plane to the higher frequency side by diminishing the size of the conductor plane. However, in practical cases, conductor planes of the same electric potential need to be DC connected with each other. Because widening a part for the connection causes the conductor planes not to be small in size, the connection part needs to be narrow. However, narrowing the connection part increases the self-inductance of the corresponding part, which results in increase in a voltage drop at a time of current inflow due to switching. Therefore, there is a practical limit on diminishing the size of the conductor planes.
As a method for solving those problems, for example, one described in Patent Literature 1 will be mentioned. A structure described in Patent Literature 1 is one having an EBG (electromagnetic bandgap) characteristic (hereafter, described as an EBG structure), which is aimed at suppressing propagation of electromagnetic wave noise between power planes. By using the EBG structure, an electromagnetic noise suppression effect can be attained in a GHz range. Further, because no manipulation of the power planes is made unlike a method of separating the conductor planes into small islands, the method of Patent Literature 1 does not cause increase in the self-inductance of the power planes.
Technologies relating to the above-described one are disclosed in Patent Literature 3 and Patent Literature 4.